Add option customPlatform

[mickkael]

Fixes #1491

Description

Add the option customPlatform to the cmd to specify a different platform while building the container with kaniko. This is not virtualization, so the specified platform must be compatible with the host. Like build arm32 on an arm64 host, or build i386 on an amd64 host.

Submitter Checklist

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review them:

• Includes unit tests
• Adds integration tests if needed.

Not sure where to put the unit tests

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- kaniko adds a new flag `--customPlatform GOOS/GOARCH` to build the container for a platform different but compatible with the build host (arm32 on an arm64 host, or build i386 on an amd64 host)

[tejal29]

Thanks you so much!!!